1. Field of the Invention
The present invention relates to an exhaust gas-purifying catalyst, and in particular to a motor vehicle exhaust gas-purifying catalyst containing a zeolite and a perovskite composite oxide.
2. Description of the Related Art
In recent years, the regulation of exhaust gas of motor vehicles is being made stricter, increasing the necessity for further decreasing the amounts of hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in the exhaust gases. Conventionally, a carried catalyst in which a noble metal (catalyst active component) such as rhodium (Rh) or palladium (Pd) is carried on a porous carrier has been widely employed as an exhaust gas-purifying catalyst for motor vehicles. Such a carried catalyst is called a three-way catalyst because it can oxidize CO and HC and reduce NOx in the exhaust gas simultaneously.
The fuel is fed in a greater amount into a motor vehicle engine during startup, leading to a smaller air-to-fuel ratio (A/F) in the exhaust gas, i.e., a so-called rich (reductive) atmosphere, resulting in increase in the amount of HC and CO in the exhaust gas. In addition, the exhaust gas-purifying function is not exhibited sufficiently during the engine startup at which the catalyst is at low temperature, resulting in decrease particularly in the HC-purifying efficiency.
Catalysts are known, which use a zeolite adsorbent that adsorbs cold HC, in order to improve the purification efficiency of the HC in cold exhaust gas emitted during motor vehicle engine startup. For example, Jpn. Pat. Appln. KOKAI Publication No. 2-56247 discloses an exhaust gas-purifying catalyst having a first catalyst layer containing a zeolite as a principal component and a second catalyst layer formed thereon containing a noble metal catalyst as a principal component. Jpn. Pat. Appln. KOKAI Publication No. 7-96183 discloses an exhaust gas-purifying catalyst of a structure in which an HC-adsorbing layer containing a zeolite as a principal component and a porous HC-oxidizing layer containing palladium oxide are laminated via a barrier layer. Further, Jpn. Pat. Appln. KOKAI Publication No. 7-148429 discloses a catalyst having an HC-adsorbing layer containing a zeolite and a catalyst metal dispersion layer formed thereon.
On the other hand, Jpn. Pat. Appln. KOKAI Publication No. 62-282642 discloses a catalyst carrying palladium as a perovskite composite oxide, as a catalyst that is suppressed in sintering of palladium under a high-temperature reductive atmosphere to enhance purification efficiency. Further, Jpn. Pat. Appln. KOKAI Publication No. 3-131342 discloses a perovskite composite oxide containing lanthanum aluminate added with platinum or palladium.
However, these prior art catalysts are still low in efficiency of purifying the HC in the cold motor vehicle exhaust gas and difficult to maintain the catalytic activity over a long period of time.
Thus, an object of the present invention is to provide an exhaust gas-purifying catalyst that is superior in efficiency of purifying the HC in the cold exhaust gas emitted during engine startup of a motor vehicle and retains its HC-purifying activity for a long period of time.